Abstract: A diffuser assembly for diffusing air into wastewater includes a plurality of diffuser elements arrayed along the bottom of a container for the wastewater. Each diffuser element has an inlet connected to a source of air under pressure and an outlet for discharging air in the form of discrete bubbles. A perforate sheet is spaced above each of the diffuser elements to capture bubbles which will tend to agglomerate by the time they reach the level of the sheet. The sheet has a plurality of polygonal openings which reform the trapped gas into discrete bubbles as the gas passes through the sheet. Additional levels of perforate sheets may be spaced above the first perforate sheet. The perforate sheets may be in the form of individual sheets each attached to a respective diffuser element, or the sheets may be part of a continuous sheet or sheets suspended across the container for the wastewater.